The present invention relates to an apparatus for tracing and identifying energized, electrical current-carrying conductors, More particularly, the present invention relates to improvements in a transmitter of this type of conductor tracer apparatus, wherein the current drawn from the conductor is more constantly regulated, and the means by which current regulation is achieved contributes to greater longevity and less heat generation in the active current regulating circuit components of the transmitter.
In a conductor tracer of the type disclosed in U.S. Pat. No. 4,491,785, a transmitter draws or absorbs a current tracing signal as a series of high frequency pulses from a conductor energized by AC energizing power from an AC power source. The current tracing signal creates a predetermined electromagnetic or magnetic field tracing signal around the conductor. The magnetic field tracing signal has a strength characteristic which is directly related to the magnitude of the current tracing signal, and has a polarity characteristic which is directly related to the direction of current flow in the conductor. A receiver of the conductor tracer detects the magnitude of the magnetic field tracing signal, and the relative magnitude is employed to distinguish the conductor carrying the current tracing signal from adjacent conductors which are not carrying the current tracing signal.
It is desirable to regulate the current drawn by the transmitter to a constant value. By maintaining the current tracing signal at a relatively constant magnitude, a relatively constant magnitude magnetic field tracing signal is created about the conductor. The resulting relatively constant magnetic field tracing signal, when detected by the receiver, makes it easier to distinguish the conductor carrying the tracing signal from adjacent conductors. Regulating the current on a conductor carrying AC energizing power requires additional considerations because the relative voltage and polarity between the conductor and ground is constantly changing and the direction of current flow reverses with each successive half cycle of the AC power.
Approaches to regulating the amount of current drawn by the transmitter to create a generally uniform strength magnetic field tracing signal are disclosed in U.S. Pat. Nos. 4,491,785 and 4,642,556. The application for a Conductor Tracer with Non-Reversing Electromagnetic Field Tracing Signal, discloses a transmitter which creates a particular type of current tracing signal that avoids the detection problems associated with reversals in the polarity of the magnetic field tracing signal with successive half cycles of the AC energizing signal.
One approach in attempting to draw a relatively constant current from a conductor energized by AC power is to utilize a variable load resistance element whose resistance varies in direct relation to the voltage on the conductor. The typical variable resistance load is an emitter-follower transistor where the emitter to collector resistance varies in relation to the voltage, to thereby attempt to limit the current drawn to a relatively constant value. When the collector to emitter resistance is high and the transistor is conducting, the transmitter must absorb considerable electrical power to keep the drawn current constant. The transistor heats up in direct relation to the power it absorbs. Excessive heat in a transistor is detrimental and will severely reduce the lifetime of the transistor. Transistors which are capable of withstanding higher heating levels are relatively expensive. Additional heat sinks and other heat dissipating equipment needed to remove the excess heat from the transistor increase the cost and size of the transmitter.
On approach to avoiding excessive transistor heating is to use a resistor as the primary load and energy absorbing element. A transistor connects the load resistor across the conductor and ground, and the load current flows through the resistor. Resistors, of course, are relatively inexpensive and are intended to absorb relatively large amounts of power without detrimental consequences. The transistor usually does not experience elevated heating because its collector to emitter resistance is low and therefore it absorbs only a very small portion of the energy absorbed from the conductor. U.S. Pat. No. 4,491,785 discloses one such arrangement.
The problems associated with the fixed resistor load technique are that the current regulation capability is reduced and the operating voltage range of the transmitter is limited. The fixed resistive load element conducts current in an amount which is primarily related to the voltage on the conductor. Since the voltage on the conductor is constantly changing, the current flow absorbed by the conductor tracer transmitter also changes, creating a correspondingly changing magnetic field tracing signal which, when detected by the receiver, makes distinguishing the relative magnitude of the tracing signal more difficult. Since the value of the fixed load resistor is also selected to inhibit destructive current flow in the transistor at high voltages, the current flow reduces to an amount which is insufficient to create an adequate magnetic field tracing signal during low voltage conditions. The effective operating voltage range of the transmitter is limited according to voltage on the conductor.
In addition to requiring relatively expensive heat resistant transistors and heat dissipating equipment, the current regulation technique of using a non-saturated emitter-follower transistor as a component of the load also does not regulate the current as constantly as is desired. It has been discovered that such a single transistor emitter-follower regulator tends to increase somewhat the regulated amount of current conducted in direct relation to the voltage on the conductor.